Dungeons that haven't been generated yet, so probably new swamps (etc) only. Anything within explored zones is out, and partially explored swamp are probably also out.

There are other mods which can force dungeons to regenerate, I don't use them so I can't vouch for how safe it is.

will find out soon. semi explored swamp near my base and the large unexplored swamp that
bonemass is shown in will be my test... just hope the people joining dont get an invalid version error message

I might be wrong, but, I'm 99% certain clients don't gen dungeons. If so clients shouldn't need the mod at all, once the server has generated the dungeon that's it - the dungeon is set.

Though I'm aware valheim does some weird stuff...